NFL players staged an unprecedented wave of protest on Sunday, beginning at Wembley Stadium in London, as Donald Trump maintained his attack on players who kneel in protest against the national anthem.

It was a climactic day for a movement that began with a single backup quarterback kneeling before a pre-season game 13 months ago and has since prompted a nation to wrestle with questions of free speech and racial justice.

Colin Kaepernick, then with the San Francisco 49ers, kneeled in an attempt to provoke debate over race and police brutality. He is now without a team but active players have followed his example. This weekend, the protest spilled into baseball: the Oakland A’s catcher Bruce Maxwell kneeled during the anthem before a game against the Texas Rangers.

Early on Sunday, shortly before his own treasury secretary insisted he was not picking a fight, Trump doubled down on his bellicose remarks at a rally in Alabama on Friday night. The president repeated his challenge to NFL team owners and encouraged fans to stop attending until the owners take action.

“If NFL fans refuse to go to games until players stop disrespecting our Flag & Country, you will see change take place fast. Fire or suspend!” Trump wrote on Twitter. “NFL attendance and ratings are WAY DOWN. Boring games yes, but many stay away because they love our country. League should back US.”

Later, at an impromptu media scrum as he headed back to Washington from his Bedminster, New Jersey golf club, Trump was asked if he was inflaming racial tensions by criticising protesting players who are almost all black.

“This has nothing to do with race,” he said. “I never said anything about race. This has nothing to do with race or anything else. This has to do with respect for our country and respect for our flag.”

Cleveland Browns players stand and kneel before their game against the Colts at Lucas Oil Stadium in Indianapolis. Photograph: Thomas J. Russo/USA Today Sports

Wembley staged the day’s first game. Around 25 players from the Jacksonville Jaguars and Baltimore Ravens kneeled. The Guardian’s Sean Ingle reported that no white players appeared to kneel but “many players, coaches and even the Jaguars’ owner Shahid Khan linked arms instead as they stood, showing unity for their black team-mates against Trump”.

Later, at MetLife Stadium in New Jersey, several Miami Dolphins wore black T-shirts supporting Kaepernick before facing the New York Jets. During the anthem, the Dolphins locked arms. The Pittsburgh Steelers decided to remain in the tunnel before their game against the Bears at Chicago’s Soldier Field.

“We’re not going to play politics,” Steelers coach Mike Tomlin told CBS. “People shouldn’t have to choose. If a guy wants to go about his normal business and participate in the anthem, he shouldn’t be forced to choose sides. If a guy feels the need to do something, he shouldn’t be separated from his team-mate who chooses not to.”

Dozens more players either kneeled or locked arms before the 1pm kickoffs. Trump tweeted: “Great solidarity for our National Anthem and for our Country. Standing with locked arms is good, kneeling is not acceptable. Bad ratings!”

The shows of protest were not confined to the teams. Musicians performing the anthem also took knees at Falcons-Lions in Detroit and Seahawks-Titans in Nashville, where both teams stayed off the field.

Trump made his initial remarks at a Friday night rally for the Republican senator Luther Strange. “Wouldn’t you love to see one of these NFL owners, when somebody disrespects our flag, to say, ‘Get that son of a bitch off the field right now. Out! He’s fired. He’s fired!’” he said.

He also complained about rules introduced to improve safety in the NFL, which has a serious problem with head injuries.

The president’s remarks and his withdrawal on Saturday of a White House invitation to the NBA-champion Golden State Warriors prompted a remarkably strong response. NBA great LeBron James called the president a “bum” while Buffalo Bills NFL running back LeSean McCoy called him an “asshole”.

NFL commissioner Roger Goodell expressed disappointment with the “divisive” comments while the New England Patriots owner Robert Kraft, a close friend of Trump, was among team owners to side with their players. In a statement, Kraft said he was “deeply disappointed” by Trump’s remarks.

“There is no greater unifier in this country than sports, and unfortunately, nothing more divisive than politics,” Kraft said.

In London, Ravens owner Steve Bisciotti said of his players and coaches: “We respect their demonstration and support them 100%.” Los Angeles Rams owner Stan Kroenke, a shareholder in Arsenal, released a statement affirming his belief in his players’ “freedom to peacefully express themselves”.

The Pittsburgh Steelers sideline is nearly empty during the playing of the national anthem at Soldier Field. Photograph: Kiichiro Sato/AP

White House officials played defense. Treasury secretary Stephen Mnuchin told CNN Trump was responding to “the NFL saying people should be able to decide what they want to do and disrespect the United States flag”.

“He thinks this is about respect for the military and so many people who put their lives at risk and what the country stands for … players have the right to free speech off the field. On the field this is about respect for lots of people.”

Marc Short, White House director of legislative affairs, told Fox News Sunday players had “a first amendment right” to protest but added: “NFL owners also have a right to fire those players.”

The Hall of Fame quarterback Kurt Warner addressed the issue on the NFL Network.

“[The flag is] a symbol of the ideals of our great country: one nation, under God, indivisible, with liberty and justice for all,” he said. “The anthem: it’s a reminder to us of those things. The president of the United States, his role is to uphold and to fight for the rights of every person, every American. And so when I heard the comments, I was so disappointed because I believe [Trump’s] comments are completely contradictory to what the flag represents.

“We have this narrative that these protests are contradictory to our flag and contradictory to our military. I don’t see them that way. I see them as complementary to the ideals to the flag, to the military and what they fought for.

“I have not heard one player that has not been more than grateful to our military. This isn’t about that at all. It’s about standing up for the ideals of the flag.”